Modern office buildings, schools, hospitals, retail and commercial establishments depend greatly upon indoor lighting for the efficient conduct of business and other operations. The intensity of the lighting required varies with the particular activity performed in the building. Retail workers require one level of lighting while office workers who do a considerable amount of reading will require a different level of lighting. In the past not a great deal of attention was given to lighting. It was thought sufficient if there was adequate lighting; that is, enough light to get the job done. Unfortunately, this practice led to having more light than was necessary or desired for some of the tasks being performed. In times past this waste of light was not deemed important; however, in the present age of energy conservation, lighting is very important and it is desirable to not waste energy on excessive lighting.
Energy management systems have been developed to assist in efficient utilization of lighting. Ideally, these systems would operate by scientifically establishing lighting needs in foot candles and monitoring the intensity of light in a given area so that the desired number of foot candles of light is available. Thus, on a bright or sunny day, less energy needs to be used for lighting than on a dark or cloudy day. These energy management systems would use some sort of sensor to detect the intensity of light in an area and use that information to apply more or less energy to the lighting or portions thereof. While the energy management control system concept seems simple and straight forward, it does have problems. As a result of these problems, present energy management systems only control lighting as an ancillary function. Their primary function is to control the operation of the HVAC system, and serve only as a time clock control for the lighting; namely, turning the lights on at the start of the day and turning the lights off at the end of the day.
One problem is that most sensors use a photoelectric cell for a light sensor, but these photoelectric cells are nonlinear devices without repeatable outputs and tend to be unreliable. They also suffer degradation with time making them unsuitable where the lighting level is critical, and they are sensitive to temperature variations. They may also have significant hysteresis and are only moderately effective in distinguishing between day and night. These characteristics make them unsuitable for control of office lighting to within a few foot candles. Accordingly, it will be appreciated that it would be highly desirable to have a sensor that is highly reliable, gives repeatable outputs, gives a linear output and is not sensitive to temperature variations.
Another problem with photoelectric cell sensors is that their sensitivity is different than the sensitivity of a human eye. Photoelectric sensors respond to light, both visible and infrared, while the human eye only responds to a spectrum of frequencies which are higher than infrared. It is therefore desirable to have a detector with the same sensitivity as the human eye. It is also desirable to have a sensor that gives a linear output so that adjustments in the lighting level can be made in the least distracting or least annoying manner.
Some sensors have been made using photovoltaic cells as photodiodes, but when used as such the output is not linearly proportional to the light level. Accordingly, it will be appreciated that it would be highly desirable to have a sensor using a photovoltaic cell to obtain an output that is linearly proportional to the light level.